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how to obtain the values of a square matrix inside a circular region of image?

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as825
as825 2013-11-16
评论： as825 2014-1-22
Suppose we find a circular roi in an image. How to obtain the values of a square matrix or square area inside that circle. give that the x and y coordinates of the circle and its radius
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Walter Roberson
Walter Roberson 2013-11-16
"Largest" in which sense? The greatest area? If so then in a circle the rectangle with the greatest area is a square.
as825
as825 2013-11-16
Greatest area.suppose the value of x and y is some value and the radius of circli is 5 . then how to find the greatest area . The aim is to embed image

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Walter Roberson
Walter Roberson 2013-11-16
d = r/sqrt(2);
xcoords = [x-d x+d x+d x-d x-d];
ycoords = [y-d y-d y+d y+d y-d];
line(xcoords, ycoords)
Now what you need to deal with is the rounding to integer coordinates. Rounding towards the center is the safest way. I would need to think more to figure out if there are circumstances under which it might then be possible to get a slightly larger area by extending one pixel further in one direction.
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Walter Roberson
Walter Roberson 2013-11-16
Extracting pixel values:
YourImage(x-d:x+d, y-d:y+d)
In terms of area, Yes, any maximal square inside the circle would do, up to the effects of area reduction due to round-off of non-integers. Circles have no inherent internal orientation, so a square aligned with the axis will have the same area (to within round-off) as a square at (say) 17.29 degrees relative to the origin.

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